Personnel lifts are presently used for a wide variety of applications. A typical personnel lift includes an aerial work platform that can be raised or lowered to position a worker at a desired height. The aerial work platform and the worker can be raised to a position where the worker can change light bulbs, work on fixtures, or paint overhead surfaces, for example.
In one personnel lift, the aerial work platform is attached to the upper end of an extendible mast and includes a personnel cage for containing a worker. The mast includes a number of extendible, nested columns mounted on a base supported by wheels. The mast and base are small so that a worker can easily roll the base to a desired location. Once the personnel lift is at the desired location, outriggers are set to stabilize the base. After the personnel lift has been adequately stabilized, a worker enters the personnel cage and operates controls to raise the aerial work platform.
The columns of prior art personnel lifts feature tracks, within which a runner on an adjacent column travels to raise or lower the aerial work platform. Each column is equipped with a plurality of rollers and/or slide surfaces that facilitate the movement of the runners within the tracks. The runners, rollers, slide surfaces, and tracks must fit precisely to permit smooth and unimpeded extension of the columns. Thus, the runner/track configuration requires a significant amount of machining. In addition, the rollers and slide surfaces must be adjusted or replaced as they wear so that the proper fit of the columns is maintained.
U.S. Pat. No. 5,203,425 to Wehmeyer et al. discloses a personnel lift device having a series of nested columns. A protrusion on one column of an adjacent pair of columns fits loosely within a substantially U-shaped slot on the adjacent column. Disposed between the protrusion and the U-shaped slot is at least one strip of low friction material. The strip of low friction material is attached to the protrusion and slides against the U-shaped slot as the columns move relative to one another. Although the strip of low friction material provides smooth sliding of one column relative to another, there is no provision for adjustment of the columns or the strip of material once the strip of material begins to wear away. Thus, after a number of cycles of the columns moving up and down relative to one another, a gap may exist between the U-shaped slot and the low friction material.
It has been found that once wear has begun, a gap tends to form between the strip of low friction material and the sliding surface of prior art mast systems of the type disclosed in Wehmeyer et al. The gap results in play between the columns that may cause the columns to lean in one direction or another. This leaning is a result of the strip of wear material wearing away, and the column shifting sideways. When the columns lean toward one side, the slide materials on that side of the column receive increased concentrated pressure. This increased pressure aggravates the problem, causing further wearing away of the strip of low friction material on the leaning side. Often, prior art personnel lifts require replacement of the wear surfaces to prevent any further leaning.
Replacement of wear surfaces in a prior art tower of nested columns often requires complete disassembly of the mast so that the wear surfaces may be accessed. This disassembly may involve hours of down time for the personnel lift, as well as costly maintenance repairs.
Another problem encountered in prior art masts is that the columns are difficult to assemble. As stated earlier, the machining of the columns must be within close tolerances, or the columns may not slide properly relative to one another. In addition, many of prior art masts utilize rollers on one column that fit within channels on an adjacent column. The rollers are made to fit snugly into the channels so as to prevent fore and aft movement of the columns. Often during assembly additional shim material must be added behind the rollers to press the front of the roller into the bottom of the channel so as to prevent side-to-side movement between the columns. Adapting the fit of the rollers so that the columns have the proper smooth movement relative to one another often requires several hours of adjustments that may include disassembly and reassembly of the columns. Thus there exists a need for a better, faster method of fitting such columns together.
In summary, there exists a need for a slide assembly for the extendible masts of personnel lifts that allows the columns of a mast to be quickly and easily aligned relative to one another and that can be easily adjusted to account for wear. The adjustment for wear of the slide assembly should not require disassembly of the mast.